The invention relates to drilling of boreholes in underground formations in search for valuable materials such as hydrocarbons. More in particular, the invention relates to a downhole motor and a method for directional drilling of such boreholes.
The expression "directional drilling of a borehole" is used in the specification and in the claims to refer to drilling of a borehole of which the direction is caused to depart at will from the vertical or from any other direction.
A means known in the art for directional drilling is a "variable bent sub". The variable bent sub is a pipe section interconnecting the lower end of a drill string and the upper end of a downhole motor which is used to drive a drill bit during drilling of a borehole. The pipe section includes a flexible joint and a remotely controlled servo-mechanism for adjusting the deflection of the flexible joint. When during drilling, if the borehole should depart from its original direction, the drill string is rotated over a finite amount to orient the pipe section in the desired direction and thereafter the servo-mechanism actuated, thereby causing the sub to deflect. On further drilling, the bent sub is maintained in the best position and a curved borehole section is drilled. This curved borehole section departs from the original direction in a predetermined direction, to wit the direction of the plane in which the variable best sub in bent. When the desired inclination of the section has been reached, the sub is stretched and a straight borehole section is thereupon drilled in a direction that is at an angle to the original direction of the hole. If desired, this straight section can again be followed by a curved section by actuating the servo-mechanism.
The variable bent sub is described in detail in U.S. Pat. No. 3,713,500 (issued: Jan. 30, 1973; inventor: Russell, M. K.).
A major disadvantage of the bent sub resides in the complexity of the servo-mechanism which is included in the pipe section for adjusting the deflection of the pipe section.